1. Field of the Invention
The present invention relates to an ophthalmic photographic apparatus that uses electronic imaging means to photograph an image of a fundus of an eye to be examined.
2. Description of the Prior Art
A portable fundus camera is well known, generally either having a construction in which a part required for photographic operation such as at least part of the optical system used for observation and photography is separate, allowing it to be carried by the examiner, or a construction in which, if possible, all the camera components are housed in a case that can be carried. Japanese Patent Laid-open Publication No. 256726/92 describes the former type of fundus camera.
Because photographic images recorded by a fundus camera are used for making medical diagnoses, the images have to be reliable. For example, it is necessary to ensure that the images that are photographed do not include images of reflected light (flares) from the anterior portion of the eye. To achieve this purpose, extensive use is made of a structure in which, in the optical system, a photographic mask is disposed at a position that is conjugate with the eye fundus and a photographic stop is disposed at a position that is conjugate with the anterior portion of the eye.
It is desirable to make the above-described portable fundus cameras as light and compact as possible. To do that, it is desirable to use a very compact optical system and reduce the number of lenses used. To satisfy those conditions, the photographic mask is located on the film plane, which is to say, the final image-formation plane. This has made it unnecessary to separately provide a fundus-conjugate position that is on the image side of the focusing lens.
In recent years, moreover, fundus cameras are being used that record the fundus image as an electronic image. Utilizing an electronic photographic system makes it possible to make the camera apparatus smaller and lighter, to the extent that all the component parts can be housed in a case that is still light and small enough to be readily carried. Imaging devices used by these electronic cameras include CCD sensors and CMOS sensors.
However, because the imaging (image-formation) plane is not on the surface of the imaging device in the case of an electronic camera, it is very difficult to arrange the photographic mask at its imaging plane. If there was a desire to employ an imaging device that included a photographic mask on its imaging plane, it would be necessary to manufacture the imaging device incorporating the required mask that met the requisite size and other criteria beforehand, which would make the camera hardware very costly.
Thus, when imaging devices are used, it is necessary to provide a fundus-conjugate position somewhere else along the photographic optical path. In such a case, it is desirable that movements of the focusing lens made for adjustment in diopter of the eye to be examined have no influence on the size of the mask image on the imaging device. A fundus-conjugate position is therefore provided on the image side of the focusing lens for the photographic mask and additionally a relay lens and condenser lens are provided to form the image on the imaging device. If, however, the mask is provided on a fundus-conjugate position somewhere else between the focusing lens and the imaging plane, it would increase the length of the optical system, which, together with the need for a relay lens, makes it more difficult to achieve the light weight and small size that are the desirable attributes for a portable fundus camera.
An object of the present invention is therefore to provide an eye fundus photographic apparatus having an optical system that includes a photographic mask and a photographic stop and that is light and compact and can be readily fabricated at a low cost.